The present invention relates generally to controlling the ignition timing of a spark ignition type internal combustion engine run on a lean air-hydrocarbon fuel mixture which is supplied with hydrogen as an auxiliary fuel in addition to the hydrocarbon fuel and particularly to a method and an apparatus in which the ignition timing of an engine of this type is controlled in accordance with the amount of hydrogen supplied to the engine.
As a solution to the problem of reducing the production of air pollutants such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in exhaust gases discharged from an internal combustion engine, it is well known in the art to set the air-hydrocarbon fuel mixture of the engine considerably lean and concurrently to supply the engine with hydrogen to complement the hydrocarbon fuel thereof. In this instance, the engine has an optimum ignition timing or advance at which the output of the engine is maximized and the air pollutant content in the engine exhaust gases is minimized and which varies with the ratio of the amount of hydrogen supplied to the engine to that of the hydrocarbon fuel, for example, gasoline, supplied thereto.
An experiment confirmed the fact that the optimum ignition advance of an internal combustion engine is reduced and increased in accordance with sensed or controlled increases and decreases in the amount of hydrogen supplied to the engine relative to that of the hydrocarbon fuel supplied thereto, respectively. The experiment particularly revealed the following facts:
(1) Optimum or ideal vacuum and centrifugal advance characteristics of the engine supplied with hydrogen are substantially similar respectively to those of an engine supplied with no hydrogen. PA1 (2) Under the condition that engine intake manifold vacuum is constant, when an air-hydrocarbon fuel ratio (A/F) of the air-hydrocarbon fuel mixture of the engine is varied by making the flow rate of hydrogen supplied to the engine constant and by varying the flow rate of hydrocarbon fuel supplied to the engine, the optimum ignition advance of the engine is increased as the air-hydrocarbon fuel ratio is increased.